Here is a method I've invented to remove gummed up piston rings on old
pistons. This works most of the time for me and I was using it with a can
of GT 85 plus a blow torch during the week on an old piston with unobtainium
piston rings... All were saved!!

OK firstly inject penetrating oil into the ring gap of the offending
piston with gummed up rings.

Next play a blow torch onto the crown lightly and hey presto, the oil
boils and pushes the rings out. Works most of the time.